A New Method for Increasing the Efficiency of a PCP - the SSG

[SpiralGroove]

Hey Guys,
It's good to see everyone on the GTA having such a good time with a new mod .
It reminds me of the first time I heard about making smoke bombs & gun powder!!  I thought, Wow you can do that? ...... now those were the days  

[oldpro]

Second effort http://i.imgur.com/y6QrAMUh.jpg. http://i.imgur.com/3mgBejKh.jpg  http://i.imgur.com/seeZLwYh.jpg  http://i.imgur.com/I5iPgydh.jpg  http://i.imgur.com/iayJKs4h.jpg I have a MUCH better way but will have to wait till tomorrow.

[oldpro]

I'm seeing some very good results only problem is to get pellets to speed requires to much spring tension in a marauder due to short hammer throw but I think if I put in my valve this will only require a 6 lb spring other than that I'm seeing very very good numbers and being able to change springs and tune externally is the bees knees. I just had 5 shots in a row with 0 fps deviation!!! BOB this is really a good mod! More work tomorrow it's beer thirty here.

[Motorhead]

I'm seeing some very good results only problem is to get pellets to speed requires to much spring tension in a marauder due to short hammer throw but I think if I put in my valve this will only require a 6 lb spring other than that I'm seeing very very good numbers and being able to change springs and tune externally is the bees knees. I just had 5 shots in a row with 0 fps deviation!!! BOB this is really a good mod! More work tomorrow it's beer thirty here.

Travis,
IMO your size & WEIGHT of stop rod is absorbing too much energy. 
Also by having smaller diameter poppet valve installed, required energy to crack it open becomes less.

Just suggesting ....

[oldpro]

I used the same size rod that Bob used 3/16. Not convinced it's the issue. I'm using a bone stock .25 shooting around 800 FPS average with a 12lb  spring at around 2600 psi. I had it at 840 before with stock spring. Still much improved over stock set up. Tomorrow I'll run a full string and post. I believe I need to polish the rod and adjuster to a very smooth finish. Or go Delren on insert like Scott suggested earlier.

[Gippeto]

Have never been one to obsess about efficiency...tune to a happy point that suites the designated task and shoot the sh1t out of it. Shot count falls where ever it does.

That said...this IS interesting.

I do have one issue with this though, and I admit it could be just me... I abhor things sticking out the back of the pressure tube into my hand...for this reason, my rva's are entirely in the tube. Had a thought while reading through the thread here and did up a quick sketch.




Al

[rsterne]

While a lighter guide can't hurt, I don't think it's critical, either.... The 2260 HPA where I got a 40% increase in shot count used a 7/32" guide about 3" long.... That would weigh 15 grams, plus the nuts, and the hammer is a stock Disco at 60 grams, so the guide assembly will be almost a quarter of the total mass the spring has to accelerate.... Since Scott is working with extremely light hammers, I can see why he wants to cut the "stop rod" (as it isn't really a guide, it's much smaller) down in weight.... Since hammer energy (lift) is a function of spring force and travel, while the hammer weight only affects the momentum (dwell), we are back to the situation of having many solutions that all work.... The cool thing is that they ALL seem to be saving air....

Al, is the grey part the hammer?.... and the guide slides down inside it?.... no reason that shouldn't work.... BTW, did you notice my QB with the cover over the back?.... nothing sticking out there....

Bob

[Rocker1]

Im starting to hate all you guys especially Bob for starting this and Lloyd for feeding the fire and Scott for doing it, That's it I do hate all you uns.  David

David,
Come on in, waters warm and beers cold.
Sittin on side line "Haten" is non productive .... Get out in the shop and Make sum thing !!!

  You can bet I want in but I have a shop full of barrels and bb guns that if I don't get done I could get hanged,   I just hate I cant play right now.  David

[lloyd-ss]

Im starting to hate all you guys especially Bob for starting this and Lloyd for feeding the fire and Scott for doing it, That's it I do hate all you uns.  David

David,
Come on in, waters warm and beers cold.
Sittin on side line "Haten" is non productive .... Get out in the shop and Make sum thing !!!

  You can bet I want in but I have a shop full of barrels and bb guns that if I don't get done I could get hanged,   I just hate I cant play right now.  David

Darn, David, sounds like you got yourself in a real pickle, ha, ha. Just how did you manage to do that??? Don't you wish you could have your clone doing all the "obligation" work for you while you got to play with the fun development stuff? Or maybe you could even work on your OWN guns?  Now that is a very novel idea!
Now you gotta stop haten us    cause we be lubbin yew! 
Lloyd

[Gippeto]

Bob....the cover sticks out more than I would like for my own use.

Rva on my qb...adjusts via hex wrench.



Al

[lloyd-ss]

Al,
That is definitely a nice clean look on your QB.  I have to admit that I don't care for the stuff moving in and out of the back end and have always just considered it a necessary evil.  Guess it should be looked at as "an opportunity for improvement. "
Lloyd

[rkr]

I'm seeing some very good results only problem is to get pellets to speed requires to much spring tension in a marauder due to short hammer throw but I think if I put in my valve this will only require a 6 lb spring other than that I'm seeing very very good numbers and being able to change springs and tune externally is the bees knees. I just had 5 shots in a row with 0 fps deviation!!! BOB this is really a good mod! More work tomorrow it's beer thirty here.

This got me thinking, it's soon beer o'clock here as well. The zero fps deviation, could it be caused by zero preload on the hammer and reduced hammer mass? Think about it, when hammer normally hits the valve it is under tension by the spring and moving fast forward. When it suddenly meets the valve stem it slows down pretty fast while still being pushed by the spring. This then should cause the hammer to rock sideways a bit causing extra friction and what not. In case of this SSG there's no spring pushing the hammer and even the mass of the hammer has been reduced. Thus the hammer should rock sideways less on impact causing less friction and perhaps more even energy transfer on the valve?

[mackeral5]

Would it be possible to make a 2 or 3 piece, telescopic spring guide?  The intention here would be to eliminate or minimize the amount of spring guide protrusion when cocked.

[Rivers3Plinker]

I'm seeing some very good results only problem is to get pellets to speed requires to much spring tension in a marauder due to short hammer throw but I think if I put in my valve this will only require a 6 lb spring other than that I'm seeing very very good numbers and being able to change springs and tune externally is the bees knees. I just had 5 shots in a row with 0 fps deviation!!! BOB this is really a good mod! More work tomorrow it's beer thirty here.

Travis,
IMO your size & WEIGHT of stop rod is absorbing too much energy. 
Also by having smaller diameter poppet valve installed, required energy to crack it open becomes less.

Just suggesting ....

I'm going to try 7075 aluminum for the stop rod for this reason. Hopefully no galling will occur but I'm going to polish the bore of the rear bolt and use graphite + moly lube and see how it goes. I have to build it first!
I don't have a heated shop. Michigan had a nice December but January looks like it's going to be less comfortable in that regard.

[rkr]

Would it be possible to make a 2 or 3 piece, telescopic spring guide?  The intention here would be to eliminate or minimize the amount of spring guide protrusion when cocked.

1 piece guide would probably be enough as those rear plugs are rather thick and having it go inside the plug increases travel. A telescopic rod attached to the rear plug would eliminate the need to have the rod protrude outside the gun. I have a design that would work in my BSA, unfortunately I don't have a lathe so implementation is still question mark.

[Bullfrog]

I've been trying to keep up with this thread to the best my primitive, non-physics-inclined mind will let me.

For those getting good results on the Mrod platform, what are you setting your hammer stroke to? Long or short?

[Motorhead]

I've been trying to keep up with this thread to the best my primitive, non-physics-inclined mind will let me.

For those getting good results on the Mrod platform, what are you setting your hammer stroke to? Long or short?

FULL stroke +
Taking the strike screw in the last .090" to flush adds even more.

[rsterne]

Very nice RVA on the QB, Al.... it is somewhat unique in that it has extra room between the back of the hammer and the rear of the tube to allow the RVA to be inside.... That is why I could easily fabricate a cover for the SSG on mine.... One thing I can see with your design, is that the entire length of the preloaded spring must be behind the hammer, none of that length can be contained inside it.... The hole in the hammer has to be the depth of the hammer stroke, plus the valve lift, plus the short stub of guide sitting in it when uncocked (as drawn)....IMO, the guide should protrude into the hammer by the amount of valve lift (when uncocked), to make sure it can't hop out of the hole and jam the hammer open.... At the back of the guide, you still have to have some way to locate it in the end cap, and adjust the spring preload and the gap between the red collar and the back of the hammer (governed by the guide length).... If you look at my drawing in Reply #105, much of the length of the spring is contained inside the hammer and the green gap adjusting nut, in fact the hammer can nearly touch the adjusting nut when cocked.... The front of your spring must start behind the back of the hammer, so the back of the spring must be further back.... I don't think you will save anything on overall length, but I do like the idea of the fixed guide, because the spring won't have to expend any energy in accelerating it....

rkr, I think that having the hammer flying free while it is opening the valve may be what is helping the ES.... In most conventional PCPs, there is enough preload on the spring that it is still in contact with the hammer throughout the valve cycle.... For instance, in a Disco, you have about 1/4" of preload, and the valve only opens 1/16-1/8".... The hammer spring is, in effect, helping to open the valve, and retarding it's closing, through the whole cycle.... I realized this when I saw what happens as soon as you eliminate the gap on a SSG and have any preload.... Since the preload is several lbs., it really slows the closing of the valve, greatly increasing the report and air use.... I would postulate, therefore, that any "normal" preload on the spring does the same thing, just more gradulally instead of like an on-off switch with the SSG.... You can further extend that reasoning to what is happening with the efficiency of the two systems when on the "knee" of the veloicyt vs. preload curve.... With a conventional setup, there is a slope to the efficiency, increasing as you decrease preload.... while with the SSG, it is pretty much either "efficient" (with a gap), or "inefficient" (without one)....

A telescoping stop rod would have to be structurally sound to resist the sudden stopping force at the bottom of the spring stroke.... If it could be done, then a good idea....

I think hard aluminum is certainly an option for the guide, as would be steel tubing, to save weight.... a tube with a bore 1/2 the OD saves 1/4 the weight but only loses a few percent of the strength....

I would think that you want the longest stroke possible in any PCP.... but then I dislike shortening the hammer stroke on any PCP except as a method of severe detuning for much lower pressures.... I think that is why the MRod has that feature....

Bob

[lloyd-ss]

...................... You can further extend that reasoning to what is happening with the efficiency of the two systems when on the "knee" of the veloicyt vs. preload curve.... With a conventional setup, there is a slope to the efficiency, increasing as you decrease preload.... while with the SSG, it is pretty much either "efficient" (with a gap), or "inefficient" (without one)....
..................................
Bob

Bob,  You hit the nail on the head with that brief summary. Very good.
Lloyd

[rsterne]

For those of you trying to wrap your brain around what is happening to the efficiency here, this drawing may help.... It is "generic", the numbers are ficticious (but representative), the purpose of the graph is to give you a visual comparision of how the various types of hammer spring setup affect the efficiency curves.... and then hopefully explain why they are the shape they are....



With a conventional, preloaded hammer spring, the spring is in contact with the hammer all the time, even when it is opening the valve.... It actually helps the hammer open the valve, and slows the poppet on the return trip.... In a stock Disco, for example there is about 1/4" of preload and the valve only opens 1/16-1/8".... As you increase the preload, the hammer spring holds the valve open longer and longer, and eventually you get to the point where the valve is still open after the pellet leaves the barrel, and the velocity no longer increases.... This is shown by the plateau in the velocity curve on the above graph.... As you continue to add preload, you continue to waste more and more air however.... The efficiency curve is typically a shallow "S" curve as shown....

If you use a shorter, stiffer spring, if it is adjusted for a gap between the spring and the hammer (let's say zero preload occurs at -3 Turns out from coil bind), from there and out, the gap means that the hammer spring is no longer pushing on the valve stem directly, it has "launched" the hammer, which then does all the work.... It is proven that this setup generally has less air-wasting hammer bounce, and better efficiency, providing there actually IS a gap.... If you increase the preload to where there is no gap, you can waste even more air than with a weaker spring, because the higher force can retard the valve closing even more.... However, for the way you would normally tune a gun, on the "knee" of the velocity curve, you tend to get better efficiency.... This setup, however, for a given average spring force, will be harder to cock, particularly at the end of the cocking stroke.... and it is still possible to get some hammer bounce, because the spring, when the hammer first strikes it, has by definition zero preload, and will give easily, storing some energy to be returned as hammer bounce....

With the SSG, when properly adjusted to have a gap between the end of the spring guide and the hammer when uncocked, the spring again does not push on the hammer while it is in contact with the valve, the hammer does all the work.... If you adjust the gap down to nothing, and then start adding preload, then there is a very large spring force against the hammer while it is opening the valve, which holds the valve open, wasting air.... The point where the gap reaches zero would be at -3 Turns out on the graph.... The velocity at which this occurs (the knee of the velocity curve) will depend on the amount of preload on the spring, and the average spring force available to launch the hammer.... You might gain a few fps by adding preload, but the amount of air wasted will NOT be worth it.... On the other hand, once there is a gap, the efficiency is very good, because the large amount of preload prevents the hammer, when it is thrown back by the valve on closing, from storing energy in the hammer spring which is then returned, causing air-wasting hammer bounce.... The gap acts very much like an "of-off" switch for the efficiency.... With a conventional setup, there is a slope to the efficiency, increasing as you decrease preload.... while with the SSG, it is pretty much either "efficient" (with a gap), or "inefficient" (without one)....

I hope this helps you understand why I'm so excited by this development....

Bob